The present invention relates to methods for forming and shaping minute surfaces with great precision. In particular, this invention may be employed to fabricate a wide variety of complex devices having intricate geometric features. The ion milling method was developed in order to manufacture surface emitting semiconductor lasers, but the technique may be utilized to efficiently and accurately mass produce a virtually infinite number of different surface features of nearly any medium on a microscopic scale.
The technical background of the present invention generally pertains to recent efforts to design and manufacture extremely small lasers from semiconductor materials. Semiconductor lasers are typically multilayered structures having dimensions measured in millionths of a meter and including different kinds of semiconductor material. One of the chief advantages of using semiconductor lasers to generate output radiation is their extraordinarily high efficiency. The various layers comprising these minute lasers are composed of chemically doped semiconductor elements or compounds. Before the doping process, semiconductor material generally contains an equal number of negative and positive particles. The doping process alters the relative number of negatively charged electrons or positively charged holes by introducing additional numbers of charged particles into the originally neutral semiconductor matrial. Regions of the laser that have been doped with extra electrons are called n-type, while those populated by a majority of holes are referred to as p-type.
The basic structure of a semiconductor laser is that of a diode, an electrical device that conducts current in only in one direction. A simple cube-shaped structure that illustrates the most fundamental semiconductor laser design is shown in FIG. 1. A diode can be formed by joining a region of n-type material with a region of p-type material. In a semiconductor laser, a relatively thin zone of material that is capable of lasing is sandwiched between the n- and p-type regions. This central zone is called the active layer. When an electrical potential is imposed across the n and p regions through metal contacts attached to the exterior faces of the laser, the electrons and holes respond to the mutually attractive electrical field that this biasing voltage creates. The particles migrate across the boundaries of the central junction into the active layer and combine with their opposites. This combination process is accompanied by the emission of laser light. The strata above and below the narrowly confined active layer have a lower index of refraction than the active layer, which means that the laser light is repeatedly reflected between the n and p regions within the active layer. The only places that are available as exits for the laser output are the peripheral edges of the active layer along the outer wall of the semiconductor laser.
Since the laser output can only radiate from a narrow stripe that extends around the mid-section of the entire structure, it is exceedingly difficult to control and use the energy produced by this very simple laser. In this embodiment, the output fans out from the cube in every direction from the plane of the active layer. The energy that is generated is weak and diluted, since the stream of light cannot be gathered into a concentrated beam that can be pointed and controlled to accomplish some task. because these laser cubes are so small, one obvious way to bolster the total energy output would be to combine them together in an array. Although an assembly of many individual cubes deployed together in a planar arrangement is an attractive alternative, the simple cube structure depicted in FIG. 1 cannot fulfill this design because most of the energy emitted by each individual laser would be directed at a neighboring cube in the two dimensional array. At best, this laser architecture may be employed to form a long row of individual cubes that would emit a wide but still relatively weak stream of laser radiation.
Over the past decade, this very simple device has been vastly improved and refined. The current generation of semiconductor laser diodes includes structures having many complex layers that are formed with a multitube of exotic techniques. Recent efforts have produced complex architectures called double and buried heterojunction designs that are fabricated using an assortment of laboratory processes. Perhaps the single most important objective of recent research in this field has been the quest to produce a two dimensional array of semiconductor lasers that emit laser output in a direction that is perpendicular to the plane of the active layer. Organizing many individual lasers that emit light from their top surfaces together in a matrix would provide a means of constructing highly powerful radiation sources. Recent experimentation has yielded semiconductor lasers that incorporate tiny mirrors oriented 45 degrees from the plane of the active layer that are capable of directing some of all of the lasers emission through apertures above the mirrors. Most of these advances utilize cleaving, wet-chemical etching, dicing, second-order grating, or mass-transport procedures that are generally difficult to perform, unreliable, and unsuitable for high volume manufacture.
The electronics industry has devoted enormous efforts in the past several years to find a solution to the long-felt need for a method of fabricating surface emitting semiconductor laser diodes. Such a method would enable not only laser manufacturers but also designers of integrated circuits to control the size and shape of sub-micron features with unprecedented accuracy. Such an advance in the technology would be a fundamental construction technique for optical computer circuits, in which photons would replace electrons as the carriers of information within complex light pathways. The ideal solution to this problem would provide a practical and efficient means for growing thousands, millions, and, perhaps, billions of lasers simultaneously layer by layer on a single wafer. This method would be equally effective in fashioning sub-micron or atomic scale features in a diverse range of media. Although a chief use would certainly include the production of lasers from semiconductor materials, the technique would be invaluable in constructing any sort of micro-miniature radiation interface, reflector, transmitter, or absorber. Virtually any surface that requires a specifically determined configuration of uniform topography could be achieved using such an invention, irrespective of whether the original medium was a semiconductor, a metal, or an active or passive optical material. The Ion Milling Method claimed in this patent application addresses these objectives and provides a solution to this long-felt need.